Dispatch will be triggered by workflow. in "Jump" function you can chech if a file is exist or not. So you can check if there is a drawing with same name in same location with part/assembly. İf not you can use "Cancel PDM Command" function to cancel and you can give user a message.

I have written attached dispatch to rename drawing with part. It reads serial from parts serial from its card and rename part and drawing. It checks if there is drawing, if yes rename drawing with same name with part.